I have a 4s that was dropped into water about a year ago. I dried it out and worked perfect till now. Now it wont stay turned on without being plugged into the mains charger so cant restore as it wont power up through usb. When it does turn on there is no signal and wifi is greyed out. When plugged in, if left alone it will restart in a loop. I can open apps but music app will not play sound. Any hope for it ? Thanks in advance

Disassemble in the attempt to clean it. Careful with cutting tiny cables and connectors.

Fix#1: Clean connectors

Go to radio shack and buy a can of Contact Cleaner, this is an aerosol bottle. Unhook all cables and clean all contacts. Use a brush if you can but if not just spray and follow can's direction.

Re-assemble see if that does the trick

Fix#2: (if fix1 not enough) clean everything.

I would use a little dish detergent 1 part to 10 parts water. Use a small size, medium stiff clean tooth brush, use spray bottle that can get the liquid into tinny crevasses. Let it sit a few minutes and immediately rinse with water, THROUGHLY. Use a can of compressed air to remove water, then let the whole thing dry for a few days.